The present invention relates to a binder for a magnetic recording medium which includes a magnetic layer containing a ferromagnetic fine powder, and also to a magnetic recording medium having on a non-magnetic support a magnetic layer made of a ferromagnetic fine powder dispersed in a binder, which has excellent electromagnetic transducing characteristics, durability and shelf stability.
Magnetic recording mediums are widely used as sound recording tapes, video tapes, floppy disks, or hard disks. A typical magnetic recording medium has a magnetic layer stacked on a non-magnetic support. The magnetic layer is made of a ferromagnetic powder dispersed in a binder.
Magnetic recording mediums are required to be on high levels in various characteristics such as electromagnetic transducing characteristics, running durability and running performance. More specifically, audio tapes for recording and reproducing music are demanded to have higher original sound reproducing capability. Video tapes are demanded to be excellent in original picture reproducing capability and other electromagnetic transducing characteristics.
Thus, magnetic recording mediums are demanded to have excellent running durability, as described above, as well as excellent electromagnetic transducing characteristics. To obtain excellent running durability, an abrasive and a lubricant are generally added to the magnetic layer.
However, it is necessary in order to obtain the required running durability by an abrasive to increase the amount of abrasive added to the magnetic layer to some extent. Consequently, the content of ferromagnetic powder lowers. When an abrasive having a relatively large particle size is used in order to obtain excellent running durability, the abrasive is likely to project excessively from the surface of the magnetic layer. Accordingly, it is likely that the improvement in the running durability by addition of an abrasive will result in deterioration of the electromagnetic transducing characteristics.
In order to improve the running durability by adding a lubricant, it is necessary to increase the amount of lubricant added. Consequently, it is likely that the binder will be plasticized and hence the durability of the magnetic layer will deteriorate.
A binder, which is one of the principal components of the magnetic layer, also performs an important roll in the improvement of the durability and electromagnetic transducing characteristics, as a matter of course. Examples of binders which have heretofore been used include vinyl chloride resins, cellulose resins, urethane resins, acrylic resins, etc. However, these conventional binders involve the problems that the resulting magnetic layer is inferior in wear resistance and that members of the magnetic tape running system are contaminated.
To solve these problems, a method wherein the hardness of the magnetic layer is increased by using a rigid binder has been adopted. However, an increase in the hardness of the magnetic layer causes the magnetic layer to become markedly brittle, which gives rise to problems such as occurrence of a dropout due to contact between the magnetic recording medium and a magnetic head, and deterioration of still characteristics.
Further, if a binder which has heretofore been employed as a binder for dispersing an abrasive and carbon black is used in a magnetic recording medium having a non-magnetic layer (intermediate layer) between a magnetic layer and a non-magnetic support, the dispersibility of an abrasive and carbon black is unsatisfactory, so that it is impossible to obtain a smooth magnetic layer having favorable surface properties as a top layer.
To solve the above-described problems, a magnetic recording medium containing a polyvinyl acetal resin material has been proposed in Japanese Patent Application Laid-Open (KOKAI) No. 01-236290 (1989). The binder disclosed in this publication is excellent in the dispersibility of a ferromagnetic powder and hence makes it possible to obtain a magnetic recording medium which is superior in electromagnetic transducing characteristics, wear resistance and heat resistance. However, polyvinyl acetal has only the OH group of vinyl alcohol as a polar group and contains no other polar group such as COOM or SO.sub.3 M and is therefore unsatisfactory in terms of the dispersibility in the case of a magnetic recording medium that uses a ferromagnetic fine powder.
Japanese Patent Application Laid-Open (KOKAI) No. 02-220221 (1990) proposes a magnetic recording medium in which the dispersibility of polyvinyl acetal is improved by using as a binder a polyvinyl acetal resin material in which an atom group containing a polar group is coupled to the main chain of polyvinyl acetal. In the proposed magnetic recording medium, the dispersibility is improved to a certain extent by introducing an atom group having a polar group to the main chain of polyvinyl acetal. However, the polar groups cannot sufficiently exhibit their ability when adsorbed on the ferromagnetic powder. Therefore, the binder disclosed in the above-mentioned publication is still unsatisfactory in terms of dispersibility.
It is an object of the present invention to provide a binder for a magnetic recording medium which is made of a polyvinyl alkylal resin improved in dispersibility.
It is another object of the present invention to provide a magnetic recording medium which is superior in electromagnetic transducing characteristics.